Architectural Design and Additive Manufacturing of Mechanical Metamaterials:A Review

Mechanical metamaterials can be defined as a class of architected materials that exhibit unprecedented mechanical properties derived from designed artificial architectures rather than their constituent materials.While macroscale and simple layouts can be realized by conventional top-down manufacturing approaches,many of the sophisticated designs at various length scales remain elusive,due to the lack of adequate manufacturing methods.Recent progress in additive manufacturing(AM)has led to the realization of a myriad of novel metamaterial concepts.AM methods capable of fabricating microscale architectures with high resolution,arbitrary complexity,and high feature fidelity have enabled the rapid development of architected meta materials and drastically reduced the design-computation and experimental-validation cycle.This paper first provides a detailed review of various topologies based on the desired mechanical properties,including stiff,strong,and auxetic(negative Poisson’s ratio)metamaterials,followed by a discussion of the AM technologies capable of fabricating these metamaterials.Finally,we discuss current challenges and recommend future directions for AM and mechanical metamaterials.

Unit Cell Modelling of Auxetic Structure

Auxetic material structures exhibit a negative Poisson ratio. The structure expands in the axial and transverse directions under tensile loading and vice versa under compression loading. Many fabricated designs for auxetic materials exist such as re-entrant hexagonal, chiral, and arrowhead geometries. This paper studies the unit cell of the re-entrant hexagonal geometry to understand how changing the internal angle and fillet radius of the structure affects the Poisson’s ratio. The material chosen for this study is acrylonitrile butadiene styrene (ABS) due to its availability and frequent use in additive manufacturing. The study was based on finite element analysis. It is observed that the direction of load applied to the unit cell affects the unit cell strain, Poisson’s ratio, and maximum load capacity before failure responses. It is noticed that the re-entrant cell starts by showing a standard non-auxetic behavior until it reaches a specific axial strain value. A quadratic correlation is identified between axial and transverse strain. Designing an auxetic structure starts with understanding the behavior of a unit cell structure. The auxetic structure design is a complex process that requires a compromise between auxetic property to be achieved and load capacity via avoiding stress concentration zones.

Emergent Physics on the Route from Crystals to Amorphous Solids

By introducing and graduly increasing the magnitude of disorder into both crystallineparticle packings and crystalline networks,we realize the evolution from crystals to amorphous solids.The evolution of structures and mechanical properties during this process is expected to help,us understand the physical origin of the peculiar properties of amorphous solids,e.gthebosonpeak,so as to buildupthebridgebetween wll-established physics of crystals and that of amorphous solids.To our surprise,this attempt also reveals some extraordinary phenomena,including the hidden order-disorder transition from crystals to disordered crystals and the emergence of mechanical metamaterials with negative Poisson's ratios.

Prediction of semiconducting SiP_(2)monolayer with negative Possion’s ratio,ultrahigh carrier mobility and CO_(2)capture ability

Using particle swarm optimization(PSO)methodology for crystal structure prediction,we predicted a novel two-dimensional(2 D)monolayer of silicide diphosphorus compound:SiP_(2),which exhibits good stability as examined via cohesive energy,mechanical criteria,molecular dynamics simulation and all positive phonon spectrum,respectively.The SiP_(2)monolayer is an indirect semiconductor with the band gap as 1.8484 eV(PBE)or 2.681 eV(HSE06),which makes it more advantageous for high-frequencyresponse optoelectronic materials.Moreover,the monolayer is a relatively hard auxetic material with negative Possion’s ratios,and also possesses a ultrahigh carrier mobility(1.069×10^(5)cm^(2)V^(-1)s^(-1))which is approximately four times the maximum value in phosphorene and comparable to the value of graphene and CP monolayers.Furthermore,the effects of strains on band structures and optical properties of SiP_(2)monolayer have been studied,as well as CO_(2)molecules can be strongly chemically adsorbed on the SiP_(2)monolayer.A semiconductor-to-metal transition for-9.5%strain ratio case and a huge optical absorption capacity on the order of 10^(6)cm^(-1)in visible region present.These theoretical findings endow SiP_(2)Monolayer to be a novel 2 D material holding great promises for applications in highperformance electronics,optoelectronics,mechanics and CO_(2)capturing material.